The present invention relates to oxidation catalysts usable in particular for the full oxidation to CO2 and H2O of volatile organic compounds (VOC) and in the processes in which said catalysts are used.
A characteristic of the catalysts is the oxidation of VOC compounds with the selective formation of carbon dioxide only. This is an evident advantage with respect to the known types of oxidation catalyst, in which the combustion of the VOC compounds is accompanied by the formation of CO, which besides being a toxic component implies an energy loss when the combustion of the VOC compounds is used to generate energy.
The known types of oxidation catalyst used for the combustion of VOC compounds are essentially of two types:
a) catalysts based on noble metals: they are characterized by high activity even at relatively low temperatures (250-450xc2x0 C.), but their cost is very high and is rising considerably owing to the scarcity of the metals and to their increasing demand, entailing problems in using them for applications such as the combustion of VOC compounds;
b) catalysts based on mixed oxides, such as copper chromites and barium hexaluminates, which are far less active than catalysts containing noble metals and require very drastic operating conditions; and catalysts based on rare earth complex oxides, alkaline-earth metals and transition metals (disclosed in U.S. Pat. No. 5,242,881) or having the formula La (1-x) Srx CrO3, the latter being also used to treat the emissions of internal-combustion engines (U.S. Pat. No. 5,286,698) or having the formula Ba2Cu3O6, which are selective in the oxidation of VOC compounds toward the formation of carbon dioxide, but are highly reactive toward CO2 and therefore tend to passivate irreversibly.
The catalysts according to the present invention comprise mixed oxides of Cu, Mn and rare-earth metals, in which the metals can assume multi-valence states, having a composition by weight expressed as the oxides that are specified hereafter: 10 to 75% as MnO, 8 to 50% as CuO, and 2 to 15% as La2O3 and/or as oxides of the other rare-earth metals at the lowest valence state.